Vitamin A (retinol) is a fat-soluble vitamin found mainly in fish liver oils, liver, egg yolk, butter, and cream. Green leafy and yellow vegetables contain beta-carotene and other provitamin carotenoids which are converted to retinol in the mucosal cells of the small intestine. Retinol cannot be synthesized in vivo and must be obtained from the diet. Retinol is metabolized into the biologically active derivative retinoic acid (RA) in a variety of cells. The 11-cis isomer of retinal (vitamin A.sub.1 aldehyde), combined with a protein moiety, forms the prosthetic group of photoreceptor pigments in the retina that are involved in night, day, and color vision. Retinol, RA, and other retinoids also influence epithelial cell differentiation.
A number of carrier proteins which bind retinol or other retinoids have been identified. These carrier proteins are similar to the fatty acid binding proteins, a family of small (14-16 kDa) cytosolic proteins which bind long-chain fatty acids, fatty acyl-coenzyme A (CoA) derivatives, and other hydrophobic molecules (Veerkamp, J. H. et al. (1991) Biochim. Biophys. Acta 108:1-24). Cellular retinol-binding proteins (CRBPs) are found within cells which participate in vitamin A metabolism or function. CRBPs, which are expressed in a variety of organs and tissues, deliver retinol to specific metabolic enzymes and to specific binding sites within the nucleus, participate in the transepithelial movement of retinol across blood-organ barriers, and are involved in the intestinal absorption of vitamin A.
Retinoid binding proteins appear to direct bound retinoid molecules to specific metabolic pathways. The retinoid binding proteins also protect the cell from the damaging effects of unliganded retinols (such as membrane disruption) and likewise protect structurally unstable retinols from non-enzymatic side reactions (such as isomerization and oxidation). Retinoid binding proteins also appear to function as sensors of retinoid concentration and act as modulators of retinoid metabolism (Napoli, J. L. (1996) FASEB J. 10:993-1001). Retinoid binding proteins have been cloned and characterized from a variety of sources including mouse and human (Smith, W. C. et al. (1991) EMBO J. 10:2223-2230; Colantuoni, V. et al. (1985) Biochem. Biophys. Res. Comm. 130:431-439).
Retinoids have been found to be effective in suppressing tumor development in several carcinogenesis model systems and in human subjects (Lotan, R. (1996) FASEB J. 10:1031-1039, and references therein). Some retinoids were found to be active in certain animal models and not in others. The effect of retinoids was not restricted to a specific carcinogen, but rather to the type of tissue involved. This restriction suggests that some retinoids exhibit tissue selectivity. Paradoxically, studies have also demonstrated that certain retinoids which are active inhibitors of carcinogenesis in particular tissues can act as enhancers of carcinogenesis in the same tissue of a different strain of mouse or in a different carcinogenesis model (Lotan, supra).
The abundance of intracellular retinoid binding proteins may play a role in the response of various tissues to retinoids. Treatment of F9 teratocarcinoma stem cells with retinoic acid (RA) causes their irreversible differentiation into extraembryonic endoderm. Boylan, J. F. et al. (1991; J. Cell. Biol. 112: 965-979) generated stably transfected F9 stem cell lines expressing either elevated or reduced levels of functional CRABP-I protein. CRABP-I is a retinol binding protein which preferentially binds retinoic acid. Cell lines expressing elevated levels of CRABP-I exhibited a significant reduction in the expression of RA-inducible mRNAs at low exogenous RA concentrations, but this reduction was eliminated at higher RA concentrations. Thus, higher levels of CRABP-I reduced the potency of RA in this differentiation system. Boylan et al. (supra) proposed that the retinol binding protein sequesters RA within the cell and thereby prevents RA from acting to regulate differentiation specific gene expression, and that the level of the binding protein may affect tissue response to RA during development.
Retinoids affect sebaceous gland activity and exhibit immunomodulatory and anti-inflammatory properties (Orfanos, C. E. et al. (1997) Drugs 53:358-388). Retinoids have been used for topical and systemic treatment of psoriasis and other hyperkeratotic and parakeratotic skin disorders, severe acne and acne-related dermatoses, and for therapy and/or chemoprevention of skin cancer and other neoplasia including T-cell lymphoma (Orfanos, et al., supra). Treatment of human pancreatic carcinoma cell lines with retinoids resulted in growth inhibition and differentiation of ductal, but not acinar, pancreatic tumor cells (Rosewicz, S. et al. (1995) Gastroenterology 109:1646-1660).
Toxic side-effects associated with retinoid treatments include changes in the skin and mucous membranes (dry skin, hair loss, dry nose, conjunctivitis), musculoskeletal symptoms, ophthalmological effects, changes in transaminase activity, changes in clinical chemistry markers (increase in serum triglycerides and decrease in high-density lipoproteins) and, rarely, central nervous system effects. A serious toxicological aspect of retinoid treatment is teratogenesis. Retinoid therapies are not recommended for women of childbearing age, and conception should be prevented for a significant period of time after stopping treatment (La Vecchia, C. et al. (1996) IARC Sci. Publ. 139:135-142)
Inadequate intake or utilization of vitamin A can impair dark adaptation and cause night blindness; xerosis of the conjunctiva and cornea; xerophthalmia and keratomalacia; keratinization of lung, GI tract, and urinary tract epithelia; and increased susceptibility to infections. Defective taste and smell, and anemia that may be masked by hemoconcentration, have also been reported (Berkow, R. and Fletcher, A. J. (1992) The Merck Manual of Diagnosis and Therapy, Merck & Co. Rahway, N.J. pp. 959-960).
Primary vitamin A deficiency is usually caused by prolonged dietary deprivation. It is endemic in areas such as southern and eastern Asia where rice, devoid of carotene, is the staple. Secondary deficiency may be due to inadequate conversion of carotene, or to interference with absorption, storage, or transport of vitamin A. Interference with absorption or storage is likely in celiac disease, sprue, cystic fibrosis, operations on the pancreas, duodenal bypass, congenital partial obstruction of the jejunum, obstruction of the bile ducts, giardiasis, and cirrhosis of the liver. Vitamin A deficiency is common in protein-energy malnutrition not only because the diet is deficient, but also because vitamin A storage and transport are defective. Liver stores are depleted in deficiency before plasma levels begin to fall, followed by retinal dysfunction, and finally by epithelial structural changes.
The discovery of a new human retinoid binding protein and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, preventions, and treatment of cancer, inflammation, and disorders associated with retinoid metabolism.